Electric cable



Jan. l2, 1937. c. J. BEAVER Er A1.`

ELECTRIC CABLE Filed Jan. 27, 1953 Sgm? Wim?

Patented Jan. 12, 1937 I ELECTRIC CABLE Charles James Beaver and EdwardLeslie Davey, Cheshire, England, assignors to W. -'1. Glover and CompanyLimited, Manchester, England,

a British company Applicants. January 27,1933, serial Nn. 653,163 InGreat Britain February 9, 1932 11 Claims.

which the cable, previously lapped with paper, is,y

dried and impregnated in vacuo with oil or compound (hereinafter, forconvenience, spoken of as compound), it is recognized that it isimpossibleto eliminate entirely residual air and gas.

Further, this method imposes comparatively low limits on the viscosityof the impregnating compound in order that it may penetrate the Wall ofdielectric.

The result is that, while the residual air or gas is usually nelydivided and uniformly distributed as very small bubbles or in solutionin the impregnating compound of the dielectric when the cable is newlymade, and the dimensions of the occlusions .are such that the gas is notionizable until much higher stresses than those corresponding to theworking voltage have been applied, the said dimensions are liable tobecome altered by aggregation after the cable has been heated, so thationization may then occur at lower stresses. 30 This aggregation ispermitted by the mobile character of the impregnating compound, which isnecessitated, as mentioned, by the requirements of penetration in theimpregnation process.

On the other hand if it is attempted to impregnate more fully, so thatresidual gas is much reduced, the dil-ation of the impregnatingcompoundunrelieved by gaseous compression in the dielectric--distendsthe lead sheath when the cable is heated under load, so that on cooling,low pressure spaces, containing gas ionizable undei-working conditions,tend to form in the dielectric.

lIn order to overcome these drawbacks of lead distension and/or gasaggregation, producing conditions under which the residual gas in animpregnated paper dielectric may become ionizable under workingconditions, in accordance with the present invention. Spaces of definiteand substantially invariable'dimensions are provided for the gas whichis retained in the cable under such a pressure (substantially aboveatmospheric pressure) having regard to the dimensions of the spaces thatit is not ionizable in the conditions under which the cable will work.Thedimensions areassured (1) by forming the spaces in the process oflapping on the paper layers so that they are determined radially by thethickness of the paper, and longitudinally by the spacing ofthellappings in each layer, and (2) by providing that these spaces aresubstantially clear of free compound; that is,rthe compound is presentonly in the paper and on its surfaces and is retained there.

In preparing the paper for use in the manufacture of a cable inaccordance with the present invention it is impregnated beforeapplication to the conductor with a compound having such a setting pointas to ensure that physical stability of the impregnated paper ismaintained up to a temperature Well above the maximum reached in theworking conditions of the cable. The impregnation may, for instance, becarried out as set out in the specification of U. S. A. Patent No.1,752,972 or of U. S. A. Patent No. 1,958,984.

Inaddition to the impregnation the paper may be provided with alubricating iilm as described in the said specications. This film ispreferably applied to one side of the paper only and is of verysmallthickness. It may, for instance, consist of a coating of petroleum jellyhaving a high viscosity and set point.

. The impregnating treatment of the cable is completed by the operationson the paper above described. I There is no subsequent addition ofcompound, either during the lapping of the paper or to the completedpaper dielectric.

An example of a cable embodying the present invention is illustrateddiagrammatically in the accompanying drawing of which Figure 1 shows atransverse section and Figure 2 shows a longitudinal section. I

In this cable the conductor l is covered with a thin metal tape or foil2 and is lapped with layers or laminae of paper strip indicated by thereference numerals 3, 4 and 5. Over the 'outside of this dielectricmaterial is placed a metallized layer 6 (preferably perforated) and overthis the lead sheath 'l is applied. To the outside of the lead sheath isapplied a metal reinforcing tape 8 and `this is protected in the usualway by layers of impregnated fabric 9. l

The metal tape 2, the metallized paper 6, the reinforcing tape 8 and thefabric layers 9 will as usual be lapped on helically. No attempt hasinite inner cylindrical surface to the dielectric layer.

Instead of using this tape the desired results can be obtained byforming (either before or after stranding) the component wires of theoutermost layer of the conductor in such a way as to give a smooth outersurface to the conductor. Alternatively a thin lead tube covering theconductor as a whole may be used as described in the yspecification ofU. S. A. Patent No. 1,256,863.

It will be seen from the drawing how the dimensions of the gas spacesI0, between adjacent turns of thepaper are xed by (1) the spacing apartof adjacent turns and (2) by the thickness of the paper laminae. Theformer determines the longitudinal width of the spaces and the latter,the radial depth thereof. The metal tape 2 forms the inner surface ofthefinnermost gas spaces. The metallized paper layer 6 forms the outersurface of the outermost gas spaces. It will be understoodv that any gaswhich may be found in spaces between the metallized paper layer 6 andthe sheath 1 will not be subject to electric stress. The same remarkapplies vto the gas between the tape 2 and the wires of the conductor iand in the interior of this conductor. y

From the preceding description of a single core cable it will be readilyrecognized how the invention may be embodied in cables having more thanone core. It will be understood that the precautions taken to secure theabsence of free compound must be extended to the material used for thefillers between the cores (where these are impregnated) as well as toany belt insulation which may be used in addition to the insulation ofthe individual cores.

Accurate predetermination of the spaces for the gas formed betweenadjacent turns of each helically applied layer of impregnated paperresults from the choice of a suitable thickness of paper and thestaggeringlof successive layers so that none of the helical spaces aredirectly superposed. This last condition can be attained, for instance,by lapping the layers of impregnated paper on the cable by means of amachine of the type in which the cable rotates and the heads from whichthe paper is paid out are stationary and the relative positions of thepaper strips accurately maintainable and under continuous observation.Such a machine is described in the specification of U. S. A. Patent No.1,591,736. It is to be understood however that the immersion of thecable in compound during lapping, which is described in thatspecification, is not to be employed in the present case. The strips orwrappings of the preimpregnated paper are lapped on the cable in airwhich' is preferably kept below, normal humidity and the cable is thensheathed with lead or lead alloy with or without reinforcement. Afterthis the air enclosed in the dielectric, which is chiefly located in thespaces provided between the turns of paper, may be evacuated andreplaced by an inert dry gas, such as nitrogen. carbon dioxide or othersuitable gas. The process or removal of air and filling with gas may befacilitated by a repeated flooding and exhausting of the gas into andfrom the cable and by the application of heat to the cableintermittently or continuously, either by external means or by thepassage of current through the conductor or conductors or by both ofthese. Finally the gas is forced in under pressure, the cable havingbeen sealed at the ends in order to retain the pressure. When the cableis in service a gas cylinder can be left connected to the cable througha non-return valve so as to maintain the pressure within the cable ifaccidental lowering should occur.

'I'he gas pressure is chosen in relation to the thickness of gas lmsformed in the dielectric, which is determined by the thickness of theindividual layers of paper, so that according to well known laws theionization point of the films of gas exceeds by a predetermined marginthe working stress in the dielectric.

Instead of replacing the air by some other gas the cable may be finallynlled with dry air under pressure, but a preliminary evacuation andheating may also be required in this case.

The choice of the radial dimensions of the gas spaces and of the gaspressure will depend upon a number of conditions. It will howevergenerally be advantageous to make the gas spaces of small radialdimensions, for instance, one or one and a half thousandths of an inch.With such dimensions and with only a comparatively moderate gaspressure, for instance, flften pounds per square inch above atmosphere,very material increase in the ionization point can be obtained in cablesconstructed in accordance with the'inven tion. It is of course possibleto use other dimensions and higher pressures, the choice of pressurebeing materially affected by the cost of strengthening or reinforcingthe sheath which it may require. This may, however, in some cases beoffset by the reduction in diameter which can be obtained by increase inpressure.

By increasing the thickness of the layers or wrappings of paper from theconductor outwards, the radial dimensions of the gas spaces may also beincreased, taking advantage of the fact that the intensity of electricstress decreases from thc surface of the conductor outwards. This isindicated diagrammatically in the drawing where three differentthicknesses of paper are intended to be indicated for the regions I, Iand I respectively. Using a gas pressure of about 100 lbs. per squareinch suitable values for the thicknesses of the paper lamina: orwrappings for this example are:-

Region 3 a thickness of 1.75 thousandths of an inch.

Region 4 a thickness of 2.5 thousandths of an inch.

Region 5 a thickness of 3.75 thousandths of an inch.

The type of cable illustrated in the drawing may be used with gaspressures up to 200 lbs per square `inch and considerably higher.

In the improved cable it is provided that the solid portion of thedielectric, namely the impregnated paper, is physically stable undermost severe working conditions and the gaseous content of the cable ispreventedfrom undergoing appreciable alteration of form, position anddimensions and therefore from becoming ionizable. Further thecompressibility provided by the gaseous content of the cable preventsthe application of excessive pressure to the reinforced sheath such aswould produce permanent distension.

stantially clear of free compound under all working conditions, whichspaces are distributed through the dielectric layer and which spaces areof increasing radial dimensions from the conductor outwards.

2. An electric cable comprising a conductor, a sheath and a dielectriclayer between the conductor and the sheath, the said layer comprisingturns of impregnated paper spaced apart to provide interveningcompound-free spaces, the thickness of the paper and the correspondingradial dimensions of the spaces increasing from the conductor outwards,and gas under pressure substantially above that of the atmospherefilling said spaces.

3. An electric cable comprising a conductor, an enclosing sheath and adielectric layer between the conductor and the sheath, said layercomprising wrapped paper impregnated with highly viscous compound, andgas at a pressure substantially above that of the atmosphere, which gasoccupies spaces in the dielectric layer of dimensions predetermined insize so as to avoid ionization of gas and which spaces are substantiallyinvariable in position and dimensions and are substantially clear offree compound under all working conditions, which Vspaces aredistributed through the dielectric layer.

4. An electric cable comprising a conductor,

an enclosing sheath and a laminated dielectric layer between theconductor and the sheath, said layer comprising wrapped paperimpregnated with highly viscous compound, and gas at a pressuresubstantially above that of the atmosphere, which gas occupies spaces inthe dielectric layer of substantially invariable dimensions which arebounded by the turns and laminae of the paper, which spaces aresubstantially clear of free compound under all working conditions.

5. An electric cable comprising a conductor, an enclosing sheath and alaminated dielectric layer between the conductor and the sheath, saidlayer comprising paper impregnated with a compound which has a highsetting temperature above the range of the working temperature of thecable and which compound is thereby retained in and confined to thepaper under all working conditions, said layer also comprising gas at apressure substantially above that of the atmosphere, which gas occupiesspaces of substantially invariable dimensions, which spaces are boundedby the turns and laminae of the paper.

6. An electric cable comprising a conductor, an enclosing sheath and adielectric layer between the conductor and the sheath, said dielectriclayer comprising wrapped paper impregnated with a highly viscouscompound, said layer having helical spaces therein of mechanicallypredetermined locations and dimensions such that the dielectric strengthof any radial dimension of said layer is predetermined, said spaceshaving gas therein at a pressure above atmospheric, the spaces beingformed between edges oi adjacent turns of paper and between surfaces ofconcentric wrappings.

7. An electric cable comprising a conductor. an enclosing sheath and a.dielectric layer between the conductor and the sheath, said'layer havinghelical spaces therein of mechanically predetermined locations anddimensions, said dielectric layer comprising wrapped paper impregnatedwith a highly viscous compound, the spaces being formed between edges of.adjacent turns of paper and between surfaces of concentric wrappings,said spaces having gas therein at a pressure above atmospheric, thedielectric strength of each part of said layer being predetermined bythe dimensions of the spaces and the gas pressure.

8. An electric cable comprising a conductor, an enclosing sheath anddielectric material between the conductor and the sheath, said materialbeing constituted by definitely spaced turns and layers of paper tape ofpredetermined thickness, which tape is fully impregnated with a highlyviscous compound which is conned to the paper under all workingconditions, and gas at a pressure substantially above that of theatmosphere occupying all the spaces between the turns and layers ofpaper tape.

9. An electric cable comprising a conductor, an enclosing sheath anddielectric material between the conductor and the sheath, said materialbeing constituted by definitely spaced turns and layers of paper tape ofpredetermined thickness, which tape is fully impregnated with oil4compound which has a setting point higher than the working temperatureof the cable, whereby the compound is coniined to the paper under allworking conditions and gas at a pressure substantially above that of theatmosphere occupying all the spaces between the turns and layers ofpaper tape.

10. An electric cable comprising a conductor, an enclosing sheath anddielectric material between the conductor Aand the sheath, said materialbeing constituted by turns and layers of paper tape with spaces ofdenite and substantially invariable dimensions between the turns and ofincreasing radial dimensions from the conductor outwards, said tapebeing fully impregnated with compound which is coniined to the paperunder all working conditions and by a gas at a pressure substantiallyabove that of the atmosphere occupying all the spaces between the turnsand layers of paper tape.

11,. An electric cable comprising a conductor, an enclosing sheath anddielectric material between the conductor and the sheath, said materialbeing constituted by turns and layers of paper tape with spaces ofdefinite and substantially invariable dimensions between the turns andof increasing radial dimensions from the conductor outwards, said tapebeing fully impregnated with oil compound which has a setting pointabove the range of the working temperature of the cable, whereby thecompound is conned to the paper under all working conditions and by agas at a pressure substantially above that of the atmosphere occupyingall the spaces between the turns and layers of paper tape.

